In recent years, shortage of manpower resulting from a declining birthrate and an aging population, as well as an increase in aging population are concerned, and development of industrial/home-use robots for compensating for these problems is expected. In particular, it is essential and important that robots have a gripping mechanism capable of gripping a variety of kinds of articles, and a joint mechanism capable of setting an intended posture to do a complicated work. Application of the gripping mechanism to a prosthetic hand for persons with hand disabilities has also been investigated in various ways.
In view of the above circumstances, some finger mechanisms for gripping an article have been proposed. FIG. 21 shows an example, wherein a finger gripping mechanism incorporated with a worm and a worm wheel is used as a prosthetic hand (see e.g. patent document 1).
Specifically, a worm wheel 93 is rotated by a worm 92 mounted at a lead end of a motor 91. Thereby, links 94 and 95 serving as a gripping finger are pivotally moved in e.g. a gripping direction. Even if a force to open up the link 95 is exerted on the link 95, there is no likelihood that the worm wheel 93 may be rotated in backward direction because of engagement with the worm 92. In this arrangement, the gripping mechanism is allowed to keep holding an article.
FIG. 22 shows an example of a finger of a gripping mechanism having an increased degree of freedom by using multiple motors (see e.g. patent document 2). In the gripping mechanism, four motors 96 through 99 are used to obtain four degrees of freedom.    Patent document 1: JP (tokuhyo) Hei 9-510128    Patent document 2: JP Hei 11-156778
The conventional gripping mechanism has the following problem. For instance, in the gripping mechanism shown in FIG. 21, since the degree of freedom is one, it is difficult to grip articles of various shapes. For instance, the link 95 serving as a finger does not fit an article of a flat plate-like shape. Accordingly, the gripping mechanism is incapable of gripping the article with a proper frictional force. In other words, in the joint mechanism shown in FIG. 21, the link 95 serving as a finger is not set in an intended posture.
On the other hand, the gripping mechanism shown in FIG. 22 has a sufficient degree of freedom for a gripping operation. However, since the number of the motors in the gripping mechanism is increased, the production cost may be increased. In other words, in the joint mechanism shown in FIG. 22, the number of drive sources may be increased, although an intended posture is obtained.